Recent laboratory findings require us to re-evaluate the contention that adult mammalian skeletal muscle is virtually incapable of regeneratig. A limited degree of regeneration occurs when muscles are excised, minced into fine fragents, and replaced in the animal. An almost complete regeneration of muscle fibers occurs if adult muscle fibers are destroyed by infiltrating the muscle with the myotoxic drug, bupivacaine. After this drug treatment, all muscle fibers become necrotic; subsequent myoblastic activity results in the formation of myotubes which, if innervated, mature into muscle fibers. Bupivacaine provides an especially useful model for the study of regeneration because the stages of myogenesis are synchronized throughout the muscle. In the present proposal the bupivacaine-induced regeneration will be used to resolve the most fundamental of the remaining questions relative to the process of muscle regeneration. Light and electron microscopical methods will be used to determine whether the myoblastic activty results from division of residual skeletal muscle nuclei, from mononucleated satellite cells, or from other cellular components of muscle. Genetic marker techniques will be used to determine whether the myotubes form by fusion of mononucleated cells (as in ontogeny) or by nuclear division. Tissue culture techniques will be used to study the role of neurotrophic influences at every stage of the regenerative process. Each of these techniques has been successfully applied to these very questions in muscle fiber ontogeny. There is no doubt that they will enable us to elucidate the similarities and the differences between prenatal development and postnatal regeneration of skeletal muscle. This information will be very important in the subsequent investigations into the technology of muscle grafting in man.